6.1 Flashcards
(28 cards)
what is benzene
benzene is a naturally occurring aromatic compound, which is very stable planar ring structure with delocalised electrons
molecular and empirical formula of benzene
CH
C6H6
what is the Kekulé structure of benzene
σ bonded ring
3 alternating π bonds
highly unsaturated (so expected to be reactive)
3 pieces of experimental evidence against the Kekulé structure of benzene
1) benzene is resistant to undergo electrophilic addition (unlike alkenes) which shows there is no true C=C bond
2) enthalpy of hydrogenation is lower than expected so more stable than expected
3) x-ray diffraction techniques showed that the bond lengths are are all equal (whereas normally single bonds are longer then double bonds, these bonds are somewhere in between)
structure of benzene
benzene has a ring of 6 σ bonds, and the p-orbitals overlap causing a delocalised electron structure.
planar structure
6 σ bonds and 6 π electrons
does benzene undergo electrophilic addition
no, electrophilic substitution instead
what catalyst is required for chlorination of benzene (halogenation)
a halogen carrier
FeCl3 or AlCl3
overall equation for chlorination of benzene (halogenation)
C6H6 + Cl2 –> C6H5Cl + HCl
3 steps in chlorination of benzene (halogenation)
(electrophile generation) Cl2 + FeCl3 FeCl4- + Cl+
(curly arrow mechanism)
(catalyst regeneration) H+ + FeCl4- –> HCl + FeCl3
reagents needed in nitration of benzene
conc. nitric acid HNO3 and conc. sulphuric acid H2SO4 (catalyst)
overall equation of nitration of benzene
C6H6 + HNO3 –> C6H5NO2 + H2O
electrophile generation step of nitration of benzene
H2SO4 + HNO3 HSO4- + H2O + NO2+
catalyst regeneration step of nitration of benzene
H+ + HSO4- –> H2SO4
why is the friedel-crafts reaction useful
allows carbon chains to be added to benzene rings which is very important in synthesis
catalyst for friedel-crafts
halogen carrier- FeCl3 or AlCl3
what is phenol?
phenol is a benzene ring with an OH group on them
is phenol more electron dense that benzene ?
yes, because the lone pair on the oxygen in a p-orbital overlaps with the p-orbitals in benzene so the oxygen electrons are (partially) delocalised into the ring.
what is different (3 things) about electrophilic substitution with phenol rather then benzene
- phenol reacts more readily then benzene so can undergo POLYsubstition (rather than mono) WITHOUT a catalyst for both halogenation and nitration
- the OH groups acts as a donating group due to the lone pair so directs the substitutes to the 2,4, (6) positions
is phenol an acid or alkali
phenol is a weak acid, and will dissolve slightly in water to form weak acidic solution
it is a stronger acid than aliphatic alcohols as the ring helps weaken the OH bond so the H is more easily lost
does phenol ungergo the normal acid reactions
acid + metal, acid +metal hydroxide, metal carbonate + acid
reacts with metals and metal hydroxides BUT NOT metal carbonates
how can you identify if you have phenol
if it has an acidic pH but NOT with metal carbonate
also a white precipitate is formed when electrophilic substitution with bromine
what is Brady’s reagent
2,4-dinitrophenylhydrazine
what is 2,4-dinitrophenylhydrazine also known as
Brady’s reagent
what is a simple chemical test with Brady’s reagent and what does it tell you
it forms an orange precipitate when reacted with aldehydes and ketones
